1. Field of the Invention
The present invention relates to a wavelength division multiplexing optical transmission apparatus, a wavelength division multiplexing optical transmission system and a wavelength division multiplexing optical transmission method for transmitting wavelength division multiplexing optical signals, and more particularly to a wavelength division multiplexing optical transmission apparatus, a wavelength division multiplexing optical transmission system and a wavelength division multiplexing optical transmission method for transmitting wavelength division multiplexing optical signals with high quality.
2. Description of Related Art
In recent years, as wavelength division multiplexing means for optical signal, apparatuses using the CWDM (Coarse Wavelength Division Multiplexing) technique have been in widespread use. In the CWDM technique, the interval between multiplexed wavelengths is as wide as 20 nm, and wavelength accuracy is as moderate as central wavelength±6.5 nm. Accordingly, the CWDM technique has an advantage of not requiring accurate wavelength control and being capable of construct a network at low cost. It is important that such CWDM apparatus can be utilized irrespective of the type of transmission line and can be smoothly upgraded to large capacity in the future.
FIG. 9 illustrates wavelength assignment of CWDM, and a characteristic of dispersion and a characteristic of group delay in each of SMF (Single Mode Fiber) and DSF (Dispersion Shifted Fiber). When SMF is used as the transmission line, a zero-dispersion wavelength is in the band of 1300 nm, so all the CWDM wavelengths are positive dispersion; thus, it is not possible that the amount of group delay is identical. On the other hand, when DSF is used as the transmission line, a zero-dispersion wavelength is in the band of 1550 nm, so negative dispersion occurs in the shorter wavelength side of CWDM and positive dispersion occurs in the longer wavelength side of CWDM. Here, when dispersion, the amount of group delay and wavelength are “D”, “A” and “λ”, respectively, then the dispersion can be expressed as wavelength differential of the amount of group delay as indicated by the following formula (1).D=dA/dλ  (1)
Consequently, wavelengths having the same absolute value of dispersion in the shorter wavelength side and longer wavelength side across the zero-dispersion wavelength of the transmission line have the identical amount of group delay. When optical signals having the identical amount of group delay are multiplexed and transmitted, transmission characteristic deterioration may occurs due to nonlinear optical effects caused by XPM (cross phase modulation) or the like.
FIGS. 10A and 10B illustrate an example of prior art's wavelength division multiplexing optical transmission system.
FIG. 10A illustrates a wavelength division multiplexing optical transmission system using the CWDM technique which performs two-way transmission in one section. The wavelength division multiplexing optical transmission system 200 includes CWDM optical transceivers 101a and 101b, CWDM optical multi/demultiplexers 102a and 102b, and a transmission line 103a. As the transmission line 103a, there is used DSF.
Optical signals having respective wavelengths outputted from the CWDM optical transceivers 101a are multiplexed by the CWDM optical multi/demultiplexer 102a. The wavelength division multiplexing optical signals thus multiplexed are transmitted via the transmission line 103a and then demultiplexed by the CWDM optical multi/demultiplexer 102b and thereafter supplied to the CWDM optical transceiver 101b. Meanwhile, optical signals having respective wavelengths outputted from the CWDM optical transceivers 101b are multiplexed by the CWDM optical multi/demultiplexer 102b. The wavelength division multiplexing optical signals thus multiplexed are transmitted via the transmission line 103a and then demultiplexed by the CWDM optical multi/demultiplexer 102a and thereafter supplied to the CWDM optical transceiver 101a. In this manner, two-way transmission is implemented.
However, the above described wavelength division multiplexing optical transmission system 200 using the prior art's CWDM technique has the following problem. That is, when transmission wavelengths are arranged, as described above, across a zero-dispersion wavelength of the transmission line (indicated by dotted lines in FIG. 10A), the amount of group delay of transmitted optical signals may be identical. Accordingly, nonlinear deterioration caused by this may occur.
Thus, as a proposal of preventing this deterioration when the transmission line 103a is DSF, there is a related document 1 (Japanese Patent Application Laid-Open No. 7-336301), for example. In this document, an attempt is made to solve the above problem by assigning the wavelengths having all the wavelength division multiplexing optical signals to either of the shorter wavelength side or longer wavelength side relative to the average zero-dispersion wavelength of the whole transmission line. However, this proposal has restrictions in that only the positive dispersion region or negative dispersion region is used (four wavelengths in the example of FIG. 10A). Consequently, there is a problem of difficulties in efficiently increasing the number of multiplexed wavelengths.
As another proposal of a prior art's wavelength division multiplexing optical transmission system, there is a related document 2 (U.S. Pat. No. 6,490,064 Patent family: JP NO. 11-17656A), for example. In the related document 2, the vicinity of 1550 nm where zero dispersion occurs in the DSF is not used, and optical signals having wavelengths from 1450 nm to 1530 nm and optical signals having wavelengths from 1570 nm to 1650 nm are transmitted in a direction opposite to each other. However, in this proposal, the wavelength band in the vicinity of 1550 nm cannot be used, so there is also a problem of difficulties in efficiently increasing the number of multiplexed wavelengths.
Thus, when it is required to increase the number of multiplexed wavelengths in the prior art's wavelength division multiplexing optical transmission system, the DWDM (Dense Wavelength Division Multiplexing) technique having a dense wavelength grid must be used.
FIG. 10B illustrates a wavelength division multiplexing optical transmission system using the DWDM technique which performs two-way transmission in one section. The wavelength division multiplexing optical transmission system 300 includes DWDM optical transceivers 101c and 101d, DWDM optical multi/demultiplexers 102c and 103d, and a transmission line 103b. As the transmission line 103b, there is used DSF.
Optical signals having respective wavelengths outputted from the DWDM optical transceivers 101c are multiplexed by the DWDM optical multi/demultiplexer 102c. The wavelength division multiplexing optical signals thus multiplexed are transmitted via the transmission line 103b and then demultiplexed by the DWDM optical multi/demultiplexer 102d and thereafter supplied to the DWDM optical transceiver 101d. Meanwhile, optical signals having respective wavelengths outputted from the DWDM optical transceivers 101d are multiplexed by the DWDM optical multi/demultiplexer 102d. The wavelength division multiplexing optical signals thus multiplexed are transmitted via the transmission line 103b and then demultiplexed by the DWDM optical multi/demultiplexer 102c and thereafter supplied to the DWDM optical transceiver 101c. In this manner, two-way transmission is implemented.
However, the above described wavelength division multiplexing optical transmission system 300 using the prior art's DWDM technique has the following problem. That is, as shown in FIG. 10B, it is required to arrange the range of multiplexed DWDM wavelengths so as not to include the zero-dispersion wavelength of transmission line, so that the amount of group delay of transmitted optical signals is not identical. Further, there is a disadvantage in that the optical multi/demultiplexer is dedicated to DWDM, so the compatibility with CWDM apparatus is not achieved and besides, a cost increases.
Further, in the wavelength division multiplexing optical transmission system, generally, dispersion tolerance is narrowed with the increase of transmission bit rate. Particularly, it is difficult to ensure dispersion tolerance at a high bit rate of 10 Gb/s or the like. It is noted here that “dispersion tolerance” means a dispersion value tolerance to guarantee the transmission quality. Thus, in the related documents 1 and 2, dispersion compensation is performed using a dispersion compensating fiber. In these documents, with the above assignment, it is attempted to ensure dispersion tolerance. However, the dispersion compensating fiber is generally costly. Also, the dispersion compensating fiber has large transmission loss; when optical signals pass through it, unwanted transmission loss occurs. Therefore, there is a problem in that the cost further increases due to additionally installed optical amplifiers.